Three officers from the Metropolitan Police's Diplomatic Protection Group have been arrested on suspicion of possessing and distributing hardcore pornography via mobile phones.

Of the three police constables from the Diplomatic Protection Group (SO6), one has been suspended while the other two are now on restricted duties. Their homes have also been searched.

They were arrested on December 19 and questioned at a central London police station, Scotland Yard said.

A Scotland Yard spokesman said: "The images identified by the investigation are of an extreme sexual nature, but do not involve children."

A fourth officer, who was later interviewed in connection with the investigation, has also been placed on restricted duties.

Police in the DPG are the officers who search visitors and vehicles heading into Downing Street and New Scotland Yard.

Current members of the Government along with former prime ministers and some former government ministers are among those who would be protected by the DPG.

The DPG is also responsible for the protection of sensitive sites such as embassies and missions along with the residential protection for visiting heads of state, heads of government and foreign ministers.

It is also involved in surveillence at events which have been identified as high risk and at diplomatic functions or key sites. These officers also provide specialist counterterrorism searches at sensitive addresses, major events and crime scenes. They also give professional security advice to the diplomatic and government communities.

The Scotland Yard spokesman said: "On 19 December 2013 officers from the DPS (Directorate of Professional Standards) arrested three police constables from the Diplomatic Protection Group (SO6) on suspicion of being involved in the possession and distribution of obscene images via mobile phones. This is contrary to both the Obscene Publications Act 1959 and also the Criminal Justice and Immigration Act 2008 (sec 63-67).

"The images identified by the investigation are of an extreme sexual nature, but do not involve children.

"The arrested officers were taken to a central London police station for questioning.

"Search warrants have been executed at the officers' home addresses.

"One officer has been suspended from duty. Two others have been placed on restricted duties.

"On 8 January 2014 a fourth officer was interviewed in connection with the investigation and placed on restricted duties.

"The MPS (Metropolitan Police Service) voluntarily referred the matter to the IPCC (Independent Police Complaints Commission) which instructed that it should be investigated locally by DPS.

"A file has now been passed to the CPS (Crown Prosecution Service) for consideration.

"The DPS misconduct investigation is on hold pending the CPS decision.

"The misconduct investigation will examine whether the conduct allegedly took place whilst the officers were on or off duty conduct."